In the simplest digital transmission link, data are sent in the form of zeros and ones that, for example, can be represented by on and off states of an oscillator signal that serves as a carrier. Turning the signal on and off can be viewed as a simple form of amplitude modulation having two states. Efficiency of the digital transmission link can be improved by using multiple states of amplitude modulation and phase modulation and, in the case of optical transmission, multiple polarization states of the carrier.
A simple form of digital phase modulation is binary phase-shift keying (BPSK) that allows two discrete states of phase of the carrier signal. A spectrally more efficient form of digital phase modulation is quadrature phase-shift keying (QPSK) in which the phase of the carrier signal can take any of four discrete states. Both BPSK and QPSK are particular kinds of n-ary phase-shift keying, a modulation format that allows n discrete states of the phase of the carrier.
Another family of digital modulation formats combines amplitude shift keying and phase shift keying. A subclass of these formats is sometimes denoted as quadrature amplitude modulation (QAM).
Regardless of the format of modulation used, signals modulated with data must be demodulated at the receiver to recover the data. Modulated signals are often received in a superheterodyne receiver having a local oscillator (LO). A signal from the LO is mixed with the received signal to generate an intermediate-frequency (IF) signal that may then be amplified and demodulated to recover the data.
Another approach is to lock the LO to the carrier frequency of the modulated signal by means of a phase-locked loop (PLL). A receiver embodying this technique is known as a homodyne receiver and can be viewed as a special case of a superheterodyne receiver in which the IF frequency is zero. In one version operating at optical frequencies, an optical modulated signal and an optical LO signal are combined and then detected in a square-law detector. The square-law detector produces a heterodyne beat signal at electrical frequencies that can be further processed to recover the data.
As the frequency of the carrier signal increases, the amount of data the signal can carry also increases, and, as a result, digital transmission has moved into optical frequency bands. Such systems use optical signals as carriers. These carriers have frequencies in the range of 200 TeraHertz (THz) and carry data at rates exceeding 100 gigabits/second (Gb/s). At these frequencies, traditional methods of demodulation, for example, methods that employ PLLs, are difficult to implement.
One solution to this problem is to use a receiver with a free-running LO. The nominal frequency of such an LO is nearly the same as that of the carrier but is not constant as the LO is not locked with a PLL and therefore the LO frequency may drift over time. The carrier frequency may also drift. The result is an IF signal with a frequency lower than the bandwidth of the modulated signal. The IF frequency is low but not constant and in general not equal to zero, and therefore such a receiver is strictly speaking neither homodyne nor heterodyne. This type of receiver is known as an intradyne receiver.
Optical oscillators, just like electrical oscillators, are not ideal. Their frequencies are not constant but exhibit some fluctuations. Since optical oscillators operate in the 200 THz range, these fluctuations can be many orders of magnitudes higher than those observed at electrical frequencies. These fluctuations are related to oscillator line width, which broadens with increasing phase noise. At optical frequencies this phase noise may become a significant issue.
As noted above, mixing an unlocked LO signal with an incoming modulated signal results in an IF signal of unknown frequency. The frequency of this IF signal must be estimated before the signal can be demodulated to recover the data. Any phase fluctuation of the IF signal must also be estimated and compensated for. An algorithm that is often used for this purpose in the case of n-ary phase-shift key modulation is the Viterbi-Viterbi algorithm. Typically this algorithm operates on frames of data (a “frame” of data is a data set of limited length) by finding a least square solution for an IF signal assumed to have a linear phase slope. However, if there is too much phase noise, frame-based processing that assumes a linear phase slope (constant IF) becomes unsuitable.
In addition to phase noise, there are other demodulation problems that have not been adequately addressed by existing methods of demodulation. For example, optical signals are subject to dispersion as they propagate through optical links, and the resulting distortion must be compensated. If two orthogonal polarization states are used for transmission, the polarization states must be identified and aligned in the receiver by means of optical or mathematical transformation of the received optical signal. Often the system clock and its phase must also be recovered from the incoming signal. Consequently, the modern optical receiver must be designed to perform a variety of functions including, among others, those listed above.
Accordingly, there has been a need for a way to demodulate a digitally-modulated optical signal.